A radar level gauge (RLG) is suitably used for making measurements of a process variable, e.g. a filling level, of a product such as process fluids, granular compounds and other materials contained in a tank, e.g. a silo.
An example of such a radar level gauge can include transceiver circuitry for transmitting and receiving microwaves, a signal propagating device arranged to direct microwaves towards the surface and to return microwaves reflected by the surface to the transceiver, and processing circuitry adapted to determine the filling level based on a relation between microwaves transmitted and received by the transceiver. The signal propagation device may be for example a directional antenna or a transmission line probe.
There are different principles for performing radar level gauges, including frequency modulated continuous wave (FMCW) and time domain reflectometry (TDR). An FMCW based RLG will emit a radar sweep with gradually varying frequency, and mix the received signal with the original signal (homodyne mixing) to form a frequency domain tank signal. A TDR based RLG will emit a pulse train of very short pulses (order of ns), and sample the received signal with the original signal in a sample and hold circuit, thereby forming a time domain tank signal.
In both cases, the tank signal will include a set of peaks indicating echoes from the tank, and one of these peaks corresponds to the echo from the surface.
The processing is configured to identify, and typically also track, the relevant echoes, in order to distinguish the surface echo from other echoes, such as reflections from structures in the tank, double bounces, etc. The surface echo can then be used to determine the distance to the surface, and consequently the filling level.
However, in some situations, it may not be possible to distinguish the surface echo in the tank signal. In the case of liquids, an undetectable echo may be caused e.g. by turbulence on the surface, or a patch of foam located underneath the antenna or around the probe, such that the echo is not strong enough. In the case of solids, e.g. grain or pellets, an undetectable echo may be caused by a surface which deviates from the horizontal plane to such an extent that the echo cannot be received by the antenna. In this case (solids) the problem may also remain for longer periods, as the slope of the surface will not change until something disturbs the product in the tank. In some cases, this will not happen until the tank is filled or emptied. In extreme cases, the surface may remain undetectable for hours or days.
In principle, an undetectable surface represents a fault condition which should be communicated e.g. by an alarm. However, for many applications, it is quite normal that the surface is undetectable at least for shorter periods of time, and it would be inconvenient to raise an alarm every time. A conventional approach to handling a temporarily undetectable surface is to let the radar level gauge enter a “wait state” for a predetermined time period. If the surface echo reappears during this time, no alarm is necessary, and the level gauging may be resumed. However, if no surface echo has reappeared at the end of the predetermined time, an alarm will be set.
A drawback with this approach is that the level gauge is “blind” during the wait state, and cannot determine if the surface is moving. Therefore, in order to maintain a high level of security, the time period cannot be set too long, and is typically set to a few minutes.